1. Field of the Invention
The present invention relates to an antiskid brake controlling method and apparatus for a vehicle for preventing locking of the wheels of the vehicle during braking of the vehicle.
2. Description of the Prior Art
In an abrupt braking of a vehicle, an excessively large braking input exerted on the vehicle wheel causes a locking of the wheel resulting in a lowered braking efficiency. In order to avoid this undesirable locking of the wheel, it is preferred to automatically control the braking torque independently of the braking input by the manual control, such that the slip rate of the wheel falls between 15 and 25%.
FIGS. 1 and 2 illustrate conventional methods of controlling the braking torque.
In the method shown in FIG. 1, the braking torque T.sub.B applied to the wheel is drastically increased from the beginning of the braking operation. In consequence, the acceleration V.sub.w of the wheel is gradually decreased. At a time t.sub.1 at which the wheel acceleration V.sub.w drops below a reference vehicle deceleration -V.sub.w3, the braking torque T.sub.B is maintained constant. Then, at a time t.sub.2 at which the wheel speed V.sub.w drops below a reference wheel speed V.sub.R to create the possibility of locking of the wheel, the braking torque T.sub.B is decreased. As a result, the wheel acceleration V.sub.w first decreases further and then increases to exceed the aforementioned reference wheel deceleration -V.sub.w3 at a time t.sub.3. The decrease of the braking torque T.sub.B is halted at this moment t.sub.3 and is then maintained constant. In consequence, the wheel acceleration V.sub.w further increases and becomes positive causing an increase in the wheel speed V.sub.w. Then, at time t.sub.5 at which the reference wheel acceleration +V.sub.w2 is exceeded by the wheel acceleration V.sub.w, the braking torque T.sub.B is increased and there is first a net increase of the wheel acceleration V.sub.w whereafter the wheel acceleration V.sub.w decreases. At a time t.sub.6 at which the wheel acceleration V.sub.w drops below the aforementioned reference wheel acceleration +V.sub.w2, the braking torque T.sub.B is maintained constant to cause the wheel acceleration V.sub.w to decrease. As this wheel acceleration V.sub.w drops below another reference wheel acceleration +V.sub.w1 at time t.sub.7, the braking torque T.sub.B is increased again. In consequence, the wheel acceleration V.sub.w is further decreased below the aforementioned reference wheel deceleration -V.sub.w3 and the braking torque T.sub.B is then maintained constant. Thereafter, this braking cycle is repeated as desired. As a consequence, the wheel speed V.sub.w is gradually decreased smoothly while repeating momentary increase and decrease as shown in FIG. 1.
According to this control method, the decrease of the braking torque is stopped at the time t.sub.3 at which the wheel acceleration V.sub.w exceeds the reference wheel deceleration -V.sub.w3, i.e. before the wheel deceleration is completely eliminated. This is undertaken in order to take into account the time lag of operation of the braking system such as the hydraulic system. Namely, at the instant at which the decrease of the braking torque is actually stopped, the braking torque T.sub.B has been decreased to such a level as to completely eliminate the wheel deceleration and, on the contrary, to generate an increase of the wheel speed.
This control method, however, imposes the following problem. In the event that the vehicle travels from a road having a surface of a high coefficient of friction to a slippery road having a low coefficient of friction during the braking, the time interval until the wheel is locked is so short that the wheel is undesirably locked before the braking torque is lowered to a sufficiently low level. As soon as the wheel is locked, the wheel deceleration is decreased to stop the decrease of the braking torque T.sub.B, so that the wheel is kept locked without allowing the recovery of the wheel speed V.sub.w. In this state, the anti-skid effect can no longer be achieved.
In order to avoid this disadvantage, it is desirable to continue the decrease of the braking torque T.sub.B until the wheel speed V.sub.w begins to be recovered, i.e. until a predetermined wheel acceleration +V.sub.w1 is obtained, even after locking of the wheel, to ensure the recovery of the wheel speed V.sub.w thereby to obtain a good antiskid effect.
In the actual braking mechanism, however, the control system as shown in FIG. 2 causes an excessive decrease of the braking torque T.sub.B which lowers the braking efficiency due to the inevitable time lag of operation. Therefore, the control system shown in FIG. 1 is generally preferred except for the special case as mentioned before.
The ordinary antiskid braking system has a hydraulic brake device including a master cylinder for generating hydraulic pressure and a wheel cylinder to which the hydraulic pressure is applied to brake the wheel. Also, a control chamber is provided to introduce working fluid under high pressure to the braking system thereby to control the operation of the wheel cylinder irrespective of the level of the hydraulic pressure generated in the master cylinder. In the case where there is a possibility of locking of the wheel due to excessively large braking force, a hydraulic control circuit detects such possibility and controls the hydraulic pressure in the control chamber to decrease the braking torque applied to the wheel or to maintain the same at a constant level. This hydraulic control circuit includes a normally closed inlet valve adapted to be switched by an inlet solenoid and disposed in a high-pressure fluid passage interconnecting the source of the high fluid pressure and the control chamber, as well as a normally closed outlet valve adapted to be switched by an outlet solenoid and disposed in a low-pressure fluid passage interconnecting the control chamber and an oil tank opened to the atmosphere.
In the antiskid brake device of the type described, the control chamber of the braking system is disconnected from the source of the hydraulic pressure but is in communication with the oil tank in the normal state in which a signal is delivered neither to the inlet solenoid nor to the outlet solenoid. In this state, the braking torque applied to the wheel is changed in accordance with the braking input of the operator. Then, when the possibility of locking of the wheel is achieved, the signal is delivered to the outlet solenoid so that the outlet valve is closed and the control chamber is isolated from the oil tank to lock the working fluid in the control chamber. Consequently, the braking torque is maintained at a constant level irrespective of the braking input of the driver. Furthermore, when both the inlet solenoid and outlet solenoid receive signals because of the possibility of locking of the wheel, the inlet valve is opened while the outlet valve is closed to permit the working fluid of high pressure to come from the hydraulic pressure source of the control circuit into the control chamber. At the same time, the control chamber is isolated from the oil tank so that the pressure in the wheel cylinder is decreased resisting the hydraulic pressure generated in the master cylinder. Namely, the braking torque is decreased irrespective of the braking input of the driver.
In the antiskid braking system of the type described, there is the problem that the hydraulic pressure in the control chamber is locked to hinder the braking operation when the low-pressure passage interconnecting the control chamber and the oil tank is clogged. To avoid this, it is necessary to select a sufficiently large cross-sectional area of the low-pressure fluid passage from being clogged. A clogging of the high-pressure fluid passage is not so serious because it does not hinder the ordinary braking function although the supply of the control hydraulic pressure to the control chamber is interrupted. However, the antiskid braking function is eliminated in this case, and hence it is preferred to provide a sufficiently large cross-sectional area for the high-pressure fluid passage as well.
An increase of the cross-sectional area in the high-pressure fluid passage and low-pressure fluid passage causes a large rate of pressure rise and drop in the control chamber. Assuming that the time lag of operation of the whole braking system is constant, the overshoots of the decrease and increase of the braking power are increased to amplify the vibration of the vehicle chassis resulting in a deteriorated braking sensation.